You start the engine and the compression will blow it out the exhaust. Will make quite a smoke show!
What I typically do for my spring decarboning is,
start up the engine and let it run until it is off the choke and running smooth, with the air cleaner off, start lightly spraying the power foam into the carburetor so the engine stumbles but does not stall. Keep spraying progressively heavier until the engine stalls out. Let it soak a half an hour to an hour and refire the engine. Get ready for a smoke show, make sure you do this outside!
Just don't go crazy with it and hydro lock the engine. It is tough to hydro lock with Power foam since it is a foam and not a straight liquid.
As an alternative, you can remove the spark plug and spray directly into the combustion chamber, get the engine as close to top dead center as you can, and it won't take much foam to completely fill the combustion chamber. Just be sure after letting it soak to crank it over with the starter a few times with the spark plug still removed to blow out the excess and prevent hydro lock.
Spraying into a running engine will clean the carburetor throat and butterfly and the back side of the intake valve as well.